This invention relates to a control system for an AC elevator on the basis of thyristor control of a three-phase induction motor.
FIG. 7 shows a block diagram of a conventional AC elevator speed control system as disclosed in U.S. Pat. No. 4,491,197, including a cage 1, a counterweight 2, a main cable 3, a drive sheave 4, a three-phase induction motor 5, upward-drive electromagnetic contacts 6a, 6b, downward-drive electromagnetic contacts 7a, 7b, drive contacts 8, a tachometer generator 15, a speed command generator 16, an adder 17, a firing circuit 18, three-phase AC power source terminals R, S, T, a first pair of reversed polarity thyristors 21, 22 interposed between power source terminal R and contact 6a, a second pair of reversed polarity thyristors 23, 24 similarly interposed between power source terminal S and contact 6b, a third thyristor 25 connected between power source terminal R and contact 6b, and a fourth thyristor 26 connected between power source terminal S and contact 6a.
In operation, when upward driving is started contacts 6a, 6b and 8 are closed. The firing circuit 18 controls the firing angles or phases of the first and second pairs of thyristors 21-24 and does not fire the third and fourth thyristors 25, 26. This results in the circuit of FIG. 8a.
When the cage 1 arrives at a deceleration command point A (FIG. 9), contacts 8 are opened. When an error signal V.sub.e becomes negative, the firing circuit 18 stops controlling the firing phases of thyristors 21, 23 and instead controls the firing phases of thyristors 22 and 24-26. This results in the circuit of FIG. 8b. During deceleration, thyristors 24 and 26 in the circuit of FIG. 8b are always rendered conductive, during both upward and downward driving, as it is desirable for the flywheel current flowing through motor 5 to also flow through thyristors 24 and 26. That is, when all of the thyristors 22 and 24-26 are controlled with reference to their firing phases, a current I.sub.1 flows intermittently through motor 5 as shown in FIG. 8c, while the flywheel current is a more continuous current I.sub.2 with less ripples. As a result, fluctuations in the torque of the motor 5 are reduced, a more comfortable cage ride is provided, and the noise produced by the motor is reduced.
It should be noted that the first and second pairs of thyristors 21, 22 and 23, 24 may be bidirectional thyristors.
FIG. 9 shows the currents which the respective thyristors carry during acceleration and deceleration. As will be obvious from this Figure, the current share greatly varies from one thyristor to another. Thus, if thyristors having the same rating as thyristors 24 and 26 are used, the other thyristors will have an excessive thermal capacity and be unduly costly. If the ratings of the thyristors are determined individually in accordance with their current shares, however, the number of components would increase and fabrication would be undesirably inefficient. In FIG. 9 the slanted lines show the full firing periods of the thyristors being controlled, the uppermost indication is a speed curve, and the remaining indications show the current intervals shared by the other thyristors.
With such a conventional control system some of the thyristors are used for both power running and DC damping force generation, and as a result the currents shared by the individual thyristors are not equal and the ratings of the thyristors cannot be selected from an economical standpoint. In this system the interposed thyristors are used for only two phases of the motor, and when the phases of the thyristors are controlled when the cage is started, unbalanced currents flow through the motor and motor noise increases as well as energy consumption. In addition, upon the generation of a deceleration command the mechanical contacts 8 are opened to switch the motor to single-phase power running, and delayed contact operation occurs. This results in an uncomfortable cage ride.